The evolution of battery charging for mobile applications has been pushing a greater number of features and requirements into battery chargers that are inside a portable (mobile) device, such as a smartphone. Next generation batteries offer greater capacities and extended voltage operating ranges. As Li-Ion battery chemistries improve and the maximum battery voltage rises and approaches 5V, this may mandate chargers which are capable of boosting the input voltage, e.g., 5V, at a Universal Serial Bus (USB) connector (which voltage is provided by an external AC-DC power adapter, for example) in order to fully charge such a battery. Additionally, in the presence of high input path or cable resistances, even traditional Li-Ion batteries can have their charge times dramatically increased if only 5V are available for charging.
Also, as the minimum battery voltage falls below the minimum usable voltage required by system components within the portable device, additional voltage boost circuitry will be needed to power these loads from the battery, and prevent brown outs when the battery is at low states of charge.
Portable devices may also need to have a popular feature referred to as USB On the Go (OTG) support, which enables the portable device to act as a host when an accessory device is attached to its USB connector, including for purposes of providing power to the attached accessory device. This mandates yet a third boost circuit inside the portable device, in order to provide a steady 5V DC to an attached, USB-compatible accessory. In the case of a USB-C specification connector, the boost circuit in the portable device needs to provide a regulated 5V for compatibility.
Furthermore, larger battery capacities require greater charging currents in order to keep portable device charge times sufficiently short (and therefore acceptable). This makes the charger circuitry more sensitive to input resistance (charging cable voltage drops) and available input power.
In parallel to all of these system demands it is desirable to minimize the size of the battery charger circuitry in order to offer the smallest and lightest possible portable device.